Calcium & Magnesium with LED grow lights
Some LED growers add extra Calcium and Magnesium to their plant feed. This is partly related to grow room temperatures and heat emission. Good quality LED grow lights produce less heat than HPS, with less plant transpiration (moisture loss) as a result.
This can mean reduced mobility for Calcium and Magnesium ions in your cannabis plant. One way to avoid potential Calcium/Magnesium deficiency is simply to add extra in with your normal plant feed.
Most grow shops sell Calcium/Magnesium supplements. Calcium/Magnesium deficiencies tended to be most common with some of the older LED grow lights, and especially some of the lower quality versions. This suggests that the cal/mag deficiency may be related to e.g. an incomplete light spectrum.
With good quality modern LED grow lights the cal/mag deficiencies have not been as obvious.